Portable base station network for local differential gnss corrections

ABSTRACT

A DGNSS-based guidance system, wherein a rover receiver first utilizes data from a master base station transceiver, a DGNSS reference network, or some other differential source to compute a differentially corrected location to establish a reference DGNSS relationship. Using this location and data observed only at the rover, the rover computes an internal set of differential corrections, which set is stored in computer memory, updated as necessary, and applied in future times to correct observations taken by the rover. As the rover enters into areas of other base station receiver reference networks, the rover transceiver will send positional information it receives from the master base station to the new, secondary base station. The secondary base station then calibrates its own reference information using information sent from the original master base station.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority in U.S. Provisional Patent ApplicationNo. 61/415,693, filed Nov. 19, 2010, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Global Navigation SatelliteSystem (GNSS) base stations, and in particular to calibrating a networkof portable, secondary base or reference stations in relation to amaster base station for differential GNSS correction over a coverageregion comprising multiple overlapping coverage polygons.

2. Description of the Related Art

Differential GNSS techniques have been successfully applied for a numberof years. These techniques, for example, enable accurate real-timepositioning of a rover receiver relative to a base receiver. Thispositioning includes code-only or carrier-smoothed-code differentialtechniques that result in sub-meter accuracy, such as those employedwhile operating with older RTCM 104 messages. They include carrier phasebased techniques that facilitate centimeter-level real-time kinematic(RTK) positioning.

Differential GNSS (DGNSS), as its name implies, requires that data bedifferenced. One of the most useful differences in DGNSS, and thereforea widely used difference, is that of differencing two similarobservations of satellite ranging signals where one observation is madeat a base or reference GNSS receiver and another is made at a rover GNSSreceiver. This type of difference, commonly referred to as thesingle-difference, removes common-mode errors (i.e. errors seen by bothbase and rover receivers) such as satellite orbit errors, satelliteclock errors, and atmospheric errors that arise as the signal passesthrough the ionosphere and the troposphere. The remaining sources oferror that result when employing single-difference techniques are thosethat are unique to the receiver, such as receiver noise and multipath.These remaining errors are often small in comparison to the common-modeerrors, especially when carrier-phase observations are employed. Leftunchecked, these errors can, over time, result in relatively largeinaccuracies in the differential signals provided by base stations. InDGPS/DGNSS systems, a stationary base receiver uses its known locationas a reference for computing differential corrections that correcterrors in its own phase observations, and these corrections are thensupplied over a communication link to a rover to correct similar errorsseen at the rover. Alternatively, the base station supplies itsreference location and phase observations over the communication link tothe rover for computing the differential corrections itself, or takes amathematically equivalent approach of forming single-differences betweenbase and rover observations.

It is common practice to obtain differential position information usingbase stations to provide the additional signals to GNSS satellitepositional signals. These base stations typically use either known GNSSposition coordinates selected from a list of previously used coordinatesor automatically-generated coordinates calculated by averagingGNSS-based position data. However, when a master and multiple secondarybase stations are used for covering a large region, one base station'scoordinate system may not line up with another base station's system.Thus, when a working vehicle travels from one base station signalcoverage area to another, the positions computed by a guidance CPUwithin the working vehicle may suddenly “jump” when transitioning to thecoverage area of the new GNSS base station. The vehicle (e.g.,tractor/implement) may be guided along a guide path based on the newbase station differential positioning data which does not line up withthe previous path, resulting in uneven rows within a field.

To address this problem, Whitehead and McClure U.S. Pat. No. 7,400,294,which is assigned to a common assignee and is incorporated herein byreference, discloses portable reference stations for local differentialGPS corrections. A base or reference station location(s) is determinedand stored. Either differential correction terms or raw satellite rangesare transmitted to GNSS-equipped remote or rover vehicles for use indifferential GNSS (DGNSS) positioning and guidance operations. The baseor reference station can be removed and later returned to the generalarea of its previous location, for which the GNSS position coordinateshave been saved. If placed within a predetermined threshold distance ofa previously-saved location, the base or reference station will “snap”to the saved reference location and compute GNSS corrector terms usingthe saved reference location data. Otherwise a new GNSS-based referenceposition will be computed for immediate use and added to the list ofstored reference locations for future use.

What is needed then is a system and method designed to coordinateseveral base stations together to form one large region of differentialguidance based off of a single “known” coordinate for a master basestation. Doing this would ensure that when the working vehicle leavesthe signal area of one base station and enters into another, the currentpath that the vehicle is traveling will not deviate or “jump” because ofa difference in the “known” coordinates that the base station is basingits differential positional information upon.

DGNSS requires rover and base GNSS receivers. The base is typicallystationary at a known location and sends to a rover GNSS receiver phase(or pseudo-range) observations plus its known location, or in lieu ofthis, differential correctors or other differential enabling data viaradios or cell phones. The rover receives the correctors from thebase(s) and uses them to correct its own satellite ranging signalobservations to increase their accuracy. The result is that the rovercan provide a more accurate location using corrected observations, evento the centimeter level or less when carrier phase is used in an RTKsolution. The range of a base signal is, however, limited. For largefields or tracts of land, a master and multiple secondary base receiversmay be needed to provide differential positioning data to the rovers tocover the entire region.

DGNSS base stations are used to generate and transmit GNSS correctionsfrom base reference positions to rover DGNSS receivers, typically byradio or cellular telecommunication, to improve accuracy to RTK or nearRTK (e.g., sub-centimeter) levels. Corrections are sent out from a basesystem, referenced to the coordinates set for the base, and eithermanually entered, automatically selected from a list of previously savedpoints, or automatically selected after a period of data averaging.

Most applications do not need absolute accuracy, but do require highrelative accuracy during field operations, both within the same week andfor subsequent field operations months or even years later. Inagriculture, this allows for planting, cultivating, and harvesting withminimal field disturbance for water and soil conservation, including theability to drive without damage to sub-surface drip tape used for highefficiency watering.

This invention allows a vehicle to base its entire preplanned path uponthe known coordinates of a single master base station, and then transferthose coordinates to calibrate other base stations as it enters thesignal area of these secondary base stations, thus expanding thesecondary base station network. Essentially, this invention willcalibrate secondary base stations to the coordinate information of asingle, master base station using a mobile transceiver device within aworking vehicle.

SUMMARY OF THE INVENTION

In the practice of an aspect of the present invention, a master basestation receiver can be either arbitrarily placed for relativepositioning or surveyed-in for absolute positioning. The master basestation references a master set of coordinates (XYZ), which can bemanually entered, automatically selected from a list of previously savedcoordinates or automatically selected after a period of data averaging.

A rover receives differential correction signals (DGNSS) from the masterbase station over a radio link for relatively precise guidance, forexample centimeter-level using real-time kinematic (RTK) techniques. Asthe rover enters the coverage area of a secondary base station, newcorrection signals are received from the secondary base station, whichcan differ substantially (e.g., 1 m or more) from the master basestation correction signals. Such offsets are typical with base stationsreceiving correction signals from the Wide Area Augmentation System(WAAS) and other satellite-based augmentation systems (SBAS). Theseoffsets cause an apparent GNSS-based position jump when the roverswitches to the frequency of the secondary base station upon enteringits coverage area. Merely averaging the inconsistent position correctorsfrom the master and second base stations can still result indiscrepancies of half a meter or more.

With the present invention the rover transmits these offsets over itsradio link to the secondary base station for application to itscorrection data, which will then be conformed to align with the masterbase station correction data, thus eliminating thebase-station-transition correction data jump. Correction data from themaster and the secondary base stations thus align relatively precisely,thereby enabling seamless centimeter-level guidance as the rover travelsfrom the master to the secondary base station coverage area.

A differentially-corrected-position network of base stations withoverlapping coverage polygons can thus be expanded from the originalmaster base station by computing the ΔX (latitude in degrees or meters),ΔY (longitude in degrees or meters) and ΔZ (height in meters) offsetsfrom the master coordinates for each secondary base station and applyingthese shifts at each secondary base station whereby the GNSS-basedposition discrepancies or “jumps” encountered while transitioningbetween base stations are minimized. The master-to-secondary positioninginformation shifts can either be manually entered at each secondary basestation or automatically uploaded from the rover system, e.g. as therover transitions between coverage areas around respective basestations. The rover thus functions to expand the network of overlapping,secondary base station coverage polygons as it traverses a working areaand transmits shift-based data for conforming the correctors emanatingfrom the secondary base stations.

The rover positioning system processor can define a list of basestations and their respective coverage area polygons for automaticallyswitching the rover receiver frequencies to receive the correctionsignals from the base station(s) within range. The hysteresis effectmaintains the rover receiver in contact with a current base stationuntil the rover exits its coverage area polygon, whereupon correctiondata is received from another base station, to which the rover hastransmitted the applicable master-to-secondary location coordinateoffsets over the radio link. Likewise, the hysteresis effect willmaintain the radio contact with the secondary base station until therover leaves its coverage area, whereupon coverage from the master (oranother secondary) base station seamlessly resumes.

The system can also save previously-generated secondary base locationcorrections for use in future operations, as described in U.S. Pat. No.7,400,294, which is incorporated herein by reference. For example, if asecondary base receiver antenna is returned to within a defined distance(e.g., 5 m) from a previously-generated, saved secondary base stationlocation, the secondary base location will be generated by averagingGNSS locations at that point, and corrections will be generated fromrover-transmitted master base station correction data, as describedabove. An extra message will be transmitted from the secondary basestation to the rover with the saved, previously-generated, secondarybase location corrections. Saved base station locations can thus bereused for improving rover location repeatability for accurate relativepositioning from previous DGNSS-guided operations within the coveragearea.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an aerial view of a control region with a master base GNSStransceiver and multiple secondary base GNSS transceivers each defininga respective signal coverage polygon.

FIG. 2A is an aerial view of the master base station signal coveragepolygon, showing a GNSS-equipped rover unit comprising a tractor andreceiving differential GNSS correction terms.

FIG. 2B is an aerial view showing master and secondary base stationsignal coverage polygons with the rover unit located in an overlappingcoverage area.

FIG. 2C is an aerial view showing master and secondary base stationsignal coverage polygons with the rover unit located in an overlappingcoverage area.

FIG. 2D is an aerial view showing a positioning signal shift or jumpbetween the master and secondary base station positioning signals usedfor locating the secondary base station in relation to the master basestation.

FIG. 2E is an aerial view showing the rover unit (tractor) reenteringthe master base station coverage polygon and toggling between the masterand secondary base stations for differential correction terms.

FIG. 3 is an aerial view showing the reuse of a saved secondary basestation location, including snapping to saved coordinates upon detectionwithin a predetermined radius of the saved location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Introduction andEnvironment

As required, detailed aspects of the disclosed subject matter aredisclosed herein; however, it is to be understood that the disclosedaspects are merely exemplary of the invention, which may be embodied invarious forms. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims and as a representative basis for teaching oneskilled in the art how to variously employ the present invention invirtually any appropriately detailed structure.

Certain terminology will be used in the following description forconvenience in reference only and will not be limiting. For example, up,down, front, back, right and left refer to the invention as orientatedin the view being referred to. Said terminology will include the wordsspecifically mentioned, derivatives thereof and words of similarmeaning.

Generally, a preferred embodiment of the present invention consists ofequipping a vehicle with a rover guidance unit and situating a number ofbase station transceiver units over an area to be traversed. One basestation will be identified as a “master” base station and should bepositioned relatively near the vehicle starting position. The basestations and rover receiver should be programmed to track the same GNSSconstellation(s), such as the global positioning system (GPS), Galileo,GLONASS, etc.

II. Portable Reference Station System 2

Referring to the drawings more detail, the reference numeral 10generally designates a portable reference station system embodying thepresent invention and generally including a master base or referencestation (base 1), multiple secondary base stations (bases 2-5) locatedin relation to the master base station 1, and a rover 11.

As shown in FIG. 1, a control region 12 generally includes multiplesignal coverage areas 14.1-14.5 forming polygons around perspective basestations 1-5 each broadcasting differential correction terms at arespective, unique frequency. Overlaps 16 are formed by two or moreoverlapping coverage areas 14.1-14.5. An obstruction 18 is shown incoverage area 14.2 and could comprise a hill, a treeline, a structure orany other signal-blocking obstruction casting a signal shadow into ablocked sub-area 20. The base stations are preferably located to providecomplete coverage of the control region with overlapping signal coverageareas whereby the rover 11 is always receiving GNSS differentialcorrection signals from at least one base station, in addition to directsatellite ranging transmissions.

FIG. 2A shows system 10 initialization with the rover 11 traversingcoverage area 14.1 and receiving GNSS-based guidance from the masterbase station 1. Without limitation on the generality of useful GNSSreceiver units, the base station 1 can include a smart antenna unit 22as shown in U.S. Patent Application Ser. No. 61/377,355, which isassigned to a common assignee herewith and is incorporated by reference.The smart antenna unit 22 includes a base antenna 23, a base GNSSreceiver 24 and a base CPU 25. Also without limitation, GNSS/RTKguidance and control systems and methods are shown in U.S. PatentPublication No. 2009/0164067, which is assigned to a common assigneeherewith and is also incorporated by reference. The system 10 applies anXYZ offset to a data stream sent from a master base station smartantenna unit 22 to the rover smart antenna unit 26, which includes arover antenna 27, a rover transceiver 28 and a rover CPU 30. This offsetcan be applied in real time either to the final position solution fromthe rover DGNSS or to the application using the rover DGNSS data. Theterms X, Y, and Z can be in distance or degrees for latitude andlongitude, and distances for altitude or any other combination ofmeasurements.

The base station 1 is preferably installed at a predetermined locationdesignated P.1, which can comprise a benchmark or monument for absolutepositioning and can be located (i.e. “surveyed in”) by conventionalsurveying techniques. For relative positioning, the location of P.1 canvary whereby the locations of the secondary base stations 2-5 areestablished relative to the location of base 1 at P.1. AdditionalGNSS-defined locations (e.g., P.2-P.4) can be established and recordedthroughout the coverage area 14.1.

The rover 11 is positioned over P.4, which can comprise a“point-of-interest” on the ground, and is located by 2D, planarorthogonal coordinates X.1, Y.1 relative to the base station 1. Thevertical axis Z coordinate at P.4 can be computed from Z.B.1 (at basestation 1) and Z.R (at rover 11), and would be relatively constant onflat ground whereas the XY coordinates would be variable with the movingrover 11. Various GNSS-based positioning and guidance techniques can beutilized in connection with operating the rover 11, such as tiltcompensation and articulated implement guidance and control. Moreover,the system 10 enables computing the locations of the smart antenna units22, 26 as the points-of-interest, as well as ground locations formingguide paths and the locations of other objects, such as towedimplements, in fixed and variable relations to the smart antenna units22, 26.

FIG. 2B shows the rover 11 in an overlapping coverage area 16 andreceiving GNSS differential correction terms from base stations 1 and 2.The rover antennae unit 26 can toggle signal reception between the basestations 1 and 2 automatically, e.g., based on signal strength, or theoperator can manually select differential correctors from among the basestations within range of the rover 11.

FIG. 2C shows the rover 11 computing and uploading XYZ positioningsignal shifts between base 1 and base 2. Such shifts or offsets canoccur due to various contributing factors, such as satellite signalinterference, multipath, atmospheric conditions, hysteresis, etc. Therover 11 transmits updated XYZ positioning terms (calculated relative tobase 1) to base 2 for calibrating its own GNSS-defined position.GNSS-based positions can be accurately calibrated system-wide inrelation to the master base station 1 by averaging location data fromall of the base stations, as received at the rover as it moves among thecoverage areas 14.1-14.5, and from satellite-direct rangingtransmissions, as well as other real-time and stored position correctingdata sources, including public and private subscription GNSSsatellite-based augmentation systems (SBAS). Such system-wide updatingand averaging functions tend to eliminate or at least minimizepositioning signal jumps among the coverage areas. Moreover, as such XYZcoordinate shifts or offsets are detected by the rover smart antennaunit 26, they can be uploaded to suitable telecommunications links(e.g., radio or cell phone, FIG. 2C) and used as correction terms by thebase CPU 25, or used as correction terms by the rover CPU 30. Forexample, FIG. 2D shows the XYZ shift correction terms (detected at therover 11) being automatically or manually downloaded by the base station2 for calibration relative to the master base station 1.

FIG. 2E shows the rover 11 controlled by the base 2 and reentering thecoverage area of base 1, with positioning signals available from both inthe overlap 16. As described above, the rover smart antenna unit 26 canbe programmed to update the base units providing signal coverage,whereby system-wide calibration is achieved as the rover 11 travelsamong the signal coverage areas 14.1-14.5. The rover smart antenna unit26 preferably locks onto a base until its signal is lost, and thenchecks for available differential corrector signals from other bases.The rover 11 thus transitions among the base stations on-the-fly, withminimal location computation “jumps” while transitioning among bases. Ina blocked-signal subarea, such as 20, the rover smart antennae at 26 cancontinue receiving differential correctors from another base, such asbase 5 (FIG. 1), and satellite-direct ranging transmissions.

If no base station signal is picked up by the rover smart antenna unit26, GNSS guidance can continue with satellite ranging signals receiveddirectly, but accuracy will be reduced due to the lack of thedifferential positioning component. However, the system 10 will continuechecking for base station correction signals. Once a base station signalis picked up, the rover smart antenna unit 26 will guide the rover 11along the preplanned path based on the base station signal that it picksup. To reduce this drop in signal and increase the ability tosynchronize all base stations to the master base station, additionalsecondary base stations can be added throughout the field or the regionwhere vehicle guidance is occurring. This allows for a network of basestations to be easily setup using automatic averaging and networkedtogether by the application of a series of X, Y, and Z shifts.

Changing between the use of differential corrections from each basestation in view will allow the calculation of a required X, Y, and/or Zshift from the master base station 1. These shifts will be applied tothe secondary base station(s) 2-5 to make the position transitions fromusing base corrections from the master and secondary base stationsminimized. This shift can either be entered manually at each secondarybase station, or, in the preferred embodiment, automatically uploadedfrom the rover smart antenna unit 26. Using this method, additional basestations can be included within the network to create more overlappingspace, and thus reduce transitional error. Essentially, the rover 11 isacting to synchronize the secondary base station(s) 2-5 with the masterbase station 1.

The multi-base, position-averaging calibration method described above isparticularly useful in operations, such as agriculture, requiringprecision and covering regions too large for a single base station.Moreover, agricultural and other repetitive operations can benefit fromthe repeatability of the system 10 whereby the same vehicle guide pathscan be reconstructed and followed season-after-season. For example, evenif the secondary base stations were removed during off seasons, they canbe approximately relocated and the system 10 will reconfigure itselfbased on their new positions.

FIG. 3 shows a stored location snap function for reusing saved baselocations. A base smart antenna unit 22 can be placed over a newlocation P.6. The smart antenna unit 22, which is receiving GNSS rangingsignals, will compare its new computed location to a set of storedlocations. If the new location is within a predetermined distance R,such as 5 m, of a previously-saved location, the smart antenna unit 22will “snap” to the previously-saved location and use its coordinates forcalibrating an updated location using the rover-linked, multi-baseposition averaging procedures described above.

This method of an X, Y, Z offset can be made automatic by a DGNSS basestation. At each reference point occupied when base corrections aregenerated, the base location used can be saved into the CPU memory ofthe rover 11, as well as the base station's internal memory. If the basestation is returned to within a defined distance from those series ofsaved locations, such as within five meters, the base location will begenerated from averaging DGNSS locations at that point and correctionsgenerated previously. In addition, an extra message will be transmittedoccasionally that includes the X, Y, Z error from that close, previouslyoccupied base location. This shift will be applied by the rover 11 toshift the used position to utilize the reference position of thepreviously occupied base location. This allows easy reuse of baselocations and improved repeatability of vehicle travel locations givingaccurate relative positioning from previous work carried out in thearea. This is particularly useful for farm operations where repeatingpaths benefits in lower crop damage and soil erosion.

It is to be understood that while certain aspects of the disclosedsubject matter have been shown and described, the disclosed subjectmatter is not limited thereto and encompasses various other embodimentsand aspects. The above-mentioned steps and components are not meant tolimit the use or organization of the present invention. The steps forperforming the method may be performed in any logical method and theprocess can be used for other types of processes when viable.

1. A method for differentially computing a rover unit location in adifferential Global Navigation Satellite System (GNSS) comprising thesteps of: providing a master base station with a master base GNSSantenna, a master base GNSS receiver coupled to the base GNSS antennaand a master base communication system including a master basetransmitter; providing a secondary base station with a secondary baseGNSS antenna, a secondary base GNSS receiver coupled to the secondarybase GNSS antenna and a secondary base communication system including asecondary base transmitter; providing a rover unit with a rover GNSSantenna, a rover GNSS receiver coupled to the rover GNSS antenna and arover communication system including a rover receiver; generating aposition offset corresponding to an offset between differentialcorrections created by said master base and said secondary basestations; said rover transitioning from the coverage area of one saidbase station to the coverage area of the other said base station;transmitting said position offset to said rover; and correcting saidrover GNSS-defined positioning with said position offset.
 2. The methodof claim 1, which includes the additional steps of: saving in each saidbase station multiple GNSS base locations; determining a current GNSSlocation of a respective base receiver; comparing said current GNSS basereceiver location with multiple said saved GNSS base locations toretrieve a closest saved GNSS base location: if a resultant distancebetween said current GNSS base receiver location and the closest savedGNSS base location, as determined by said comparing step, is less than asaid threshold minimum distance, identifying as a base location thesaved GNSS base location which is closest to said current GNSS basereceiver location; and otherwise identifying said current GNSS basereceiver location as the base location and saving in said base stationsaid current GNSS base receiver location.
 3. The method according toclaim 2, which includes the additional step of: reusing said positionaloffset corresponding to said previous base location for transmission tosaid rovers.
 4. The method according to claim 3, which includes theadditional steps of: marking field locations for said base stations; andremoving said base stations from said marked field locations betweenuses; and replacing said base stations at said marked field locationsfor subsequent uses.
 5. The method of claim 1, which includes theadditional step of: manually entering said offsets in said basestations.
 6. The method of claim 1, which includes the additional stepsof: automatically entering said offsets in said base stations.
 7. Themethod of claim 1, which includes the additional steps of: providing anetwork of overlapping base station coverage areas; transitioning saidrover among said base station coverage areas; and applying a respectivecoverage area-specific offset to said rover as said rover transitionsfrom one base station coverage area to another.
 8. The method of claim6, which includes the additional steps of: averaging the offsetsgenerated by said base stations; applying said averaged offsets to saidrover GNSS-defined position; and determining with said rover adifferentially-corrected GNSS-defined position using said averagedoffsets.
 9. The method of claim 1, which includes the additional stepsof: overlapping multiple said base station coverage areas within anetwork; and minimizing GNSS position transitional errors among saidbase station coverage areas.
 10. The method of claim 1, which includesthe additional steps of: receiving with said rover offsets from saidmaster base station and said overlapping secondary base station; andselecting an offset for use by said rover in an overlapping coveragearea.
 11. The method of claim 1, which includes the additional steps of:moving said rover into a blocked-signal area whereat signals from arespective base station are blocked; and receiving signals from anotherbase station in said blocked-signal area; and utilizing the offset fromsaid other base station for computing a rover GNSS-defined position insaid blocked-signal area.
 12. A method for differentially computing arover unit location in a differential Global Navigation Satellite System(GNSS) comprising the steps of: providing a master base station with amaster base GNSS antenna, a master base GNSS receiver coupled to thebase GNSS antenna and a master base communication system including amaster base transmitter; providing a secondary base station with asecondary base GNSS antenna, a secondary base GNSS receiver coupled tothe secondary base GNSS antenna and a secondary base communicationsystem including a secondary base transmitter; providing a rover unitwith a rover GNSS antenna, a rover GNSS receiver coupled to the roverGNSS antenna and a rover communication system including a roverreceiver; generating a position offset corresponding to an offsetbetween differential corrections created by said master base and saidsecondary base stations; said rover transitioning from the coverage areaof one said base station to the coverage area of the other said basestation; transmitting said position offset to said rover; correctingsaid rover GNSS-defined positioning with said position offset; saving ineach said base station multiple GNSS base locations; determining acurrent GNSS location of a respective base receiver; comparing saidcurrent GNSS base receiver location with multiple said saved GNSS baselocations to retrieve a closest saved GNSS base location: if a resultantdistance between said current GNSS base receiver location and theclosest saved GNSS base location, as determined by said comparing step,is less than a said threshold minimum distance, identifying as a baselocation the saved GNSS base location which is closest to said currentGNSS base receiver location; and otherwise identifying said current GNSSbase receiver location as the base location and saving in said basestation said current GNSS base receiver location.
 13. The methodaccording to claim 12, which includes the additional step of: reusingsaid positional offset corresponding to said previous base location fortransmission to said rovers.
 14. The method according to claim 13, whichincludes the additional steps of: marking field locations for said basestations; and removing said base stations from said marked fieldlocations between uses; and replacing said base stations at said markedfield locations for subsequent uses.
 15. The method of claim 1, whichincludes the additional step of: manually entering said offsets in saidbase stations.
 16. The method of claim 1, which includes the additionalsteps of: automatically entering said offsets in said base stations. 17.The method of claim 1, which includes the additional steps of: providinga network of overlapping base station coverage areas; transitioning saidrover among said base station coverage areas; and applying a respectivecoverage area-specific offset to said rover as said rover transitionsfrom one base station coverage area to another.
 18. The method of claim1, which includes the additional steps of: receiving with said roveroffsets from said master base station and said overlapping secondarybase station; and selecting an offset for use by said rover in anoverlapping coverage area.
 19. The method of claim 1, which includes theadditional steps of: moving said rover into a blocked-signal areawhereat signals from a respective base station are blocked; receivingsignals from another base station in said blocked-signal area; andutilizing the offset from said other base station for computing a roverGNSS-defined position in said blocked-signal area.
 20. A method fordifferentially computing a rover unit location in a differential GlobalNavigation Satellite System (GNSS) comprising the steps of: providing amaster base station with a master base GNSS antenna, a master base GNSSreceiver coupled to the base GNSS antenna and a master basecommunication system including a master base transmitter; providing asecondary base station with a secondary base GNSS antenna, a secondarybase GNSS receiver coupled to the secondary base GNSS antenna and asecondary base communication system including a secondary basetransmitter; providing a rover unit with a rover GNSS antenna, a roverGNSS receiver coupled to the rover GNSS antenna and a rovercommunication system including a rover receiver; generating a positionoffset corresponding to an offset between differential correctionscreated by said master base and said secondary base stations; said rovertransitioning from the coverage area of one said base station to thecoverage area of the other said base station; transmitting said positionoffset to said rover; correcting said rover GNSS-defined positioningwith said position offset; saving in each said base station multipleGNSS base locations; determining a current GNSS location of a respectivebase receiver; comparing said current GNSS base receiver location withmultiple said saved GNSS base locations to retrieve a closest saved GNSSbase location: if a resultant distance between said current GNSS basereceiver location and the closest saved GNSS base location, asdetermined by said comparing step, is less than a said threshold minimumdistance, identifying as a base location the saved GNSS base locationwhich is closest to said current GNSS base receiver location; otherwiseidentifying said current GNSS base receiver location as the baselocation and saving in said base station said current GNSS base receiverlocation; reusing said positional offset corresponding to said previousbase location for transmission to said rovers; marking field locationsfor said base stations; removing said base stations from said markedfield locations between uses; replacing said base stations at saidmarked field locations for subsequent uses; providing a network ofoverlapping base station coverage areas; transitioning said rover amongsaid base station coverage areas; applying a respective coveragearea-specific offset to said rover as said rover transitions from onebase station coverage area to another; receiving with said rover offsetsfrom said master base station and said overlapping secondary basestation; selecting an offset for use by said rover in an overlappingcoverage area; moving said rover into a blocked-signal area whereatsignals from a respective base station are blocked; receiving signalsfrom another base station in said blocked-signal area; and utilizing theoffset from said other base station for computing a rover GNSS-definedposition in said blocked-signal area.